For many decades, series ignition systems in present day internal combustion engines embodied as spark ignition engines have operated according to the simple and reliable principle of coil discharge, that is to say that an ignition coil correspondingly designed as a transformer is charged on the primary side in accordance with its inductance from the on board supply system voltage partly into its saturation region. At the ignition instant, the charging is interrupted by means of an electronic circuit, e.g. by an ignition IGBT (Insulated Gate Bipolar Transistor). On the secondary side, a voltage of e.g. 5 kV to 35 kV thereby builds up, which leads to a sparkover in the spark gap of the spark plug in the combustion chamber of the internal combustion engine. The energy stored in the coil subsequently decreases in the ignition plasma.
In the course of advancing engine development, savings in terms of consumption and emissions have to be realized which in recent years have consistently led to an increasing additional loading on the ignition system and will continue to do so in the future. Examples of this are e.g. charge stratification, in which liquid fuel constituents with high flow velocities impede the spark discharge and constrain numerous instances of new spark formation. Increasing combustion chamber pressures for improving engine efficiency also increase the breakdown resistance in the spark gap and constrain a rise in the breakdown voltage, which also influences spark plug wear. This last will lead, in future generations of highly charged engines, to secondary side voltage rises far beyond 35 kV. Both the increasing breakdown voltages and the flow states becoming more intensive at the spark plug tend to shorten the burning duration of the spark since higher and higher proportions of the energy stored in the coil have to be provided for establishing and maintaining the spark. One very promising trend in the development of new combustion methods is the use of multiple sparks, wherein the coil energy is efficiently transmitted to the mixture at short intervals, which increases the reliability of combustion. In the case of ignition devices currently in use, an ignition coil embodied as a transformer with magnetic storage capability is firstly charged on the primary side from the 12V on board system supply up to a current of approximately 8 A. In this case, a blocking diode fitted on the secondary side prevents undesired spark formation during the charging phase. At the ignition instant, the current flow is interrupted by means of an electronic switch—e.g. an IGBT.
The collapse of the magnetic field of the ignition coil then induces a voltage rise on the primary and secondary sides. Owing to the IGBT semiconductor technology used, the primary voltage is in this case limited to typically 400V. On the secondary side, however, the voltage obtains a significantly higher value, which is initially determined by the turns ratio of the transformer. In the case of a conventional turns ratio of 1:80, this therefore results in a maximum secondary voltage of 32 kV. This voltage is not attained in practice, however, since a voltage breakdown between the electrodes of the spark plug with a subsequent arc already takes place beforehand, whereupon the secondary voltage abruptly falls to the value of the arc burning voltage. Typical values for the breakdown voltage are 5 kV to 35 kV and depend greatly on the electrode spacing, the combustion chamber pressure and the gas temperature. The burning voltage of the arc is in the range of a few kV.
In order to attain the breakdown voltage, firstly the secondary side capacitances—caused by the spark plug and the construction of the secondary winding—have to be charged. For a given breakdown voltage Uz, the following holds true in this case:Ec=Csec*Uz2/2  {1}
Ec is the energy required for attaining the breakdown voltage, Csec is the secondarily effective capacitance.
This energy, in the case of the conventional ignition system, is supplied by the main inductance Lh of the ignition transformer, which has been correspondingly charged beforehand.El=Lh*I2/2  {2}    El is the stored energy    Lh is the main inductance of the transformer    I is the charging current
In the case of conventional ignition coils embodied as ignition transformers, the maximum stored energy is 50 mJ to 130 mJ. The residual energy available after breakdown is converted in the subsequent arc phase in the arc, the secondary current falling continuously. The burning duration of the arc of typically 0.5 ms to 1.5 ms is substantially determined by this residual energy.
The requirement for a longer burning duration—and thus increased ignition energy—in difficult combustion situations can be met by increasing the maximum stored energy. However, this necessitates enlarging the magnetic core, which leads to an undesirable enlargement of the ignition coil. Particularly in the case of so-called “pencil coils”, incorporated directly in the spark plug shaft, enlargement is not possible. A further disadvantage of simply increasing the ignition energy is the more than proportional spark plug wear associated therewith, for which reason the desired lifetime can no longer be achieved. Present day ignition systems have in some instances already reached this limit, and so simply increasing the ignition energy is not a technically expedient approach.
It has been found, however, that operating the spark plug with alternating current makes possible a lifetime two to three times longer. AC voltage ignition systems have accordingly been developed for motor vehicles. In this case, the ignition coil is embodied as a pure transformer with only low storage capability. In the case of technically expedient turns ratios of e.g. 1:100, a primary voltage of 200V is required in order to attain a breakdown voltage of e.g. 20 kV, which in turn necessitates a complex and expensive voltage converter. The high transformation ratio—from 12V on board supply system voltage to 200V ignition supply—also reduces the efficiency of the voltage converter, which in turn reduces the total efficiency of the ignition system.
Although the use of such AC voltage ignition can solve the engineering problem appertaining to combustion, for cost reasons it is only suitable for top of the range vehicles. Therefore, hitherto it has been necessary to accept the spark plug wear associated with increasing spark energy or combustion critical operating states have not been able to be realized on the series engine.